50,817 research outputs found
Marginal AMP Chain Graphs
We present a new family of models that is based on graphs that may have
undirected, directed and bidirected edges. We name these new models marginal
AMP (MAMP) chain graphs because each of them is Markov equivalent to some AMP
chain graph under marginalization of some of its nodes. However, MAMP chain
graphs do not only subsume AMP chain graphs but also multivariate regression
chain graphs. We describe global and pairwise Markov properties for MAMP chain
graphs and prove their equivalence for compositional graphoids. We also
characterize when two MAMP chain graphs are Markov equivalent.
For Gaussian probability distributions, we also show that every MAMP chain
graph is Markov equivalent to some directed and acyclic graph with
deterministic nodes under marginalization and conditioning on some of its
nodes. This is important because it implies that the independence model
represented by a MAMP chain graph can be accounted for by some data generating
process that is partially observed and has selection bias. Finally, we modify
MAMP chain graphs so that they are closed under marginalization for Gaussian
probability distributions. This is a desirable feature because it guarantees
parsimonious models under marginalization.Comment: Changes from v1 to v2: Discussion section got extended. Changes from
v2 to v3: New Sections 3 and 5. Changes from v3 to v4: Example 4 added to
discussion section. Changes from v4 to v5: None. Changes from v5 to v6: Some
minor and major errors have been corrected. The latter include the
definitions of descending route and pairwise separation base, and the proofs
of Theorems 5 and
Non-axisymmetric relativistic Bondi-Hoyle accretion onto a Schwarzschild black hole
We present the results of an exhaustive numerical study of fully relativistic
non-axisymmetric Bondi-Hoyle accretion onto a moving Schwarzschild black hole.
We have solved the equations of general relativistic hydrodynamics with a
high-resolution shock-capturing numerical scheme based on a linearized Riemann
solver. The numerical code was previously used to study axisymmetric flow
configurations past a Schwarzschild hole. We have analyzed and discussed the
flow morphology for a sample of asymptotically high Mach number models. The
results of this work reveal that initially asymptotic uniform flows always
accrete onto the hole in a stationary way which closely resembles the previous
axisymmetric patterns. This is in contrast with some Newtonian numerical
studies where violent flip-flop instabilities were found. As discussed in the
text, the reason can be found in the initial conditions used in the
relativistic regime, as they can not exactly duplicate the previous Newtonian
setups where the instability appeared. The dependence of the final solution
with the inner boundary condition as well as with the grid resolution has also
been studied. Finally, we have computed the accretion rates of mass and linear
and angular momentum.Comment: 21 pages, 13 figures, Latex, MNRAS (in press
Differences in exchange rate pass-through in the euro area.
This paper focuses on the pass-through of exchange rate changes into the prices of imports made by euro area countries originating outside the area. Using data on import unit values for thirteen different product categories for each country, we estimate industry-specific rates of pass-through across and within countries for all euro members. In the short-run, pass-through rates differ across industries and countries and are less than one. In the longrun neither full pass-through nor equality of pass-through rates across industries and countries can be rejected. Differences exist across euro area countries in the degree that a common exchange rate movement gets transmitted into consumer prices and costs of production indices. Most of these differences in transmission rates are due to the distinct degree of openness of each country to non-euro area imports rather than to the heterogeneity in the structure of imports.Exchange rate; pass-through; euro; monetary union;
Do business density and variety determine retail performance?
Outlet location plays a crucial role in retail strategy. In this paper we study the relationship between spatial density (concentration) of retailers in the trade area and their economic performance. This analysis will help managers figure out the economic potential of starting a retail business in a given area, reducing business start-up risks. We find that retail businesses located in high and low retail density zones enjoy higher performance levels, consistent with competitive advantage arising from agglomeration economies and local market power respectively. We also find that retail businesses located in intermediate density areas use a differentiation strategy based on business variety (diversification across stores). Outlets located in areas with the highest variety enjoy performance levels similar to those achieved in the agglomeration and low density areas. The results suggest that retail companies should jointly consider variety and density to determine location
General Electric-Magnetic decomposition of fields, positivity and Rainich-like conditions
We show how to generalize the classical electric-magnetic decomposition of
the Maxwell or the Weyl tensors to arbitrary fields described by tensors of any
rank in general -dimensional spacetimes of Lorentzian signature. The
properties and applications of this decomposition are reviewed. In particular,
the definition of tensors quadratic in the original fields and with important
positivity properties is given. These tensors are usually called "super-energy"
(s-e) tensors, they include the traditional energy-momentum, Bel and
Bel-Robinson tensors, and satisfy the so-called Dominant Property, which is a
straightforward generalization of the classical dominant energy condition
satisfied by well-behaved energy-momentum tensors. We prove that, in fact, any
tensor satisfying the dominant property can be decomposed as a finite sum of
the s-e tensors. Some remarks about the conservation laws derivable from s-e
tensors, with some explicit examples, are presented. Finally, we will show how
our results can be used to provide adequate generalizations of the Rainich
conditions in general dimension and for any physical field.Comment: 20 pages, no figures, plenary talks given at the Spanish Relativity
Meeting 2000, to be publisehd in the Proceedings of ERE-00 Some minor errors
corrected, one formula and acknowledgements adde
Optimal duration of magazine promotions
The planning of promotions and other marketing events frequently requires manufacturers to make decisions about the optimal duration of these activities. Yet manufacturers often lack the support tools for decision making. We assume that customer decisions at the aggregated level follow a state-dependent Markov process. On the basis of the expected economic return associated with dynamic response to stimuli, we determine the ideal length of marketing events using dynamic programming optimization and apply the model to a complex promotion event. Results suggest that this methodology could help managers in the publishing industry to plan the optimal duration of promotion event
n-ary algebras: a review with applications
This paper reviews the properties and applications of certain n-ary
generalizations of Lie algebras in a self-contained and unified way. These
generalizations are algebraic structures in which the two entries Lie bracket
has been replaced by a bracket with n entries. Each type of n-ary bracket
satisfies a specific characteristic identity which plays the r\^ole of the
Jacobi identity for Lie algebras. Particular attention will be paid to
generalized Lie algebras, which are defined by even multibrackets obtained by
antisymmetrizing the associative products of its n components and that satisfy
the generalized Jacobi identity (GJI), and to Filippov (or n-Lie) algebras,
which are defined by fully antisymmetric n-brackets that satisfy the Filippov
identity (FI). Three-Lie algebras have surfaced recently in multi-brane theory
in the context of the Bagger-Lambert-Gustavsson model. Because of this,
Filippov algebras will be discussed at length, including the cohomology
complexes that govern their central extensions and their deformations
(Whitehead's lemma extends to all semisimple n-Lie algebras). When the
skewsymmetry of the n-Lie algebra is relaxed, one is led the n-Leibniz
algebras. These will be discussed as well, since they underlie the
cohomological properties of n-Lie algebras.
The standard Poisson structure may also be extended to the n-ary case. We
shall review here the even generalized Poisson structures, whose GJI reproduces
the pattern of the generalized Lie algebras, and the Nambu-Poisson structures,
which satisfy the FI and determine Filippov algebras. Finally, the recent work
of Bagger-Lambert and Gustavsson on superconformal Chern-Simons theory will be
briefly discussed. Emphasis will be made on the appearance of the 3-Lie algebra
structure and on why the A_4 model may be formulated in terms of an ordinary
Lie algebra, and on its Nambu bracket generalization.Comment: Invited topical review for JPA Math.Theor. v2: minor changes,
references added. 120 pages, 318 reference
The -Geminid meteoroid stream: orbits, spectroscopic data and implications for its parent body
By using an array of high-sensitivity CCD video devices and spectrographs,
the activity of meteor events from the poorly-known -Geminid meteoroid
stream has been monitored during January 2012 and 2013. As a result of this
research, the atmospheric trajectory and radiant position of 10 of these events
has been obtained, but also the orbital parameters of the progenitor meteoroids
and the tensile strength of these particles. The data reveal that the
progenitor of this stream must be a comet. In addition, the emission spectra
produced by three -Geminid meteors were also recorded. These are the
first -Geminid spectra discussed in the scientific literature, and have
provided clues about the chemical nature of these meteoroids and their parent
body.Comment: Accepted for publication on Monthly Notices of the Royal Astronomical
Society on Jan. 20, 201
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